Power device and packaging thereof

ABSTRACT

The present disclosure provides a power device and power device packaging. Generally, the power device of the present disclosure includes a die backside and a die frontside. A semi-insulating substrate with epitaxial layers disposed thereon is sandwiched between the die backside and the die frontside. Pads on the die frontside are coupled to the die backside with patterned backmetals that are disposed within vias that pass through the semi-insulating substrate and epitaxial layers from the die backside to the die frontside.

RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application No. 61/693,040, filed Aug. 24, 2012, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a power device and packaging thereof and in particular to gallium nitride (GaN) power devices along with their packaging.

BACKGROUND

The packaging of power devices is critical to device and system performance. Low resistance and low inductance connections are desirable for device terminals such as the source and drain terminals of a transistor or the anode and cathode terminals of a diode.

FIG. 1 depicts a cross-section view of a prior art high-voltage transistor 10 having a vertical structure with an unpatterned backside metallization that serves as a drain pad 12. Examples of such vertical devices include double-diffused metal oxide semiconductor (DMOS) transistors, insulated gate bipolar transistors (IGBTs), and junction field effect transistors (JFETs). These devices include a die 14 with a substrate 16 and an epitaxial layer 18 that are conductive. As a result, a through wafer via is not required. Moreover, a die attach process is provided with a large area, low resistance, and high current connection to the drain pad 12. Further still, a gate bond pad 20 and a source bond pad 22 are located on a frontside 24. A gate current is typically much less than a source current. Thus, the majority of the frontside 24 is usable for the source bond pad 22. A resulting large pad area available to the source bond pad 22 enables a low-cost, high current connection using large diameter wires, ribbons, or clips. A further advantage of the large pad area is that only a few large area bonds to the source bond pad 22 are required to carry a maximum device current. Another advantage is that the die 14 is also in good thermal contact with the substrate, which assists with heat dissipation.

In contrast to vertical power devices, gallium nitride (GaN) high electron mobility transistors (HEMTs) are lateral devices. As shown in FIG. 2 depicting a bond pad layout for a GaN device 26, GaN HEMTs can have a first gate pad 28, a second gate pad 30 along with both a source pad 32 and a drain pad 34 on a die front surface 36. To minimize die area, the source pad 32 and the drain pad 34 both have dimensions that are minimized. As a result of their minimized dimensions, the source pad 32 and the drain pad 34 of GaN HEMTs only provide enough space for small diameter bond wires such as source bond wires 38 and drain bond wires 40. A typical diameter for a bond wire using gold (Au) is about 25.4 μm (1 mil). As such, a typical 1200 V class GaN device in a standard TO-247 package can require between 20 and 30 bond wires on both the source pad 32 and the drain pad 34. Not only does this many bond wires add a significant packaging cost, but bond wires have significant inductance which can have a negative effect on the switching characteristics of the GaN device 26. Larger diameter bond wires, ribbons, or clips typically require a relatively large landing area that undesirably and significantly increases total die area of the GaN device 26. Another packaging option is a flip-chip process that attaches a die to a substrate using metallic bumps that are fabricated onto bond pads. However, in this case, the die is in poor thermal contact with the substrate, which results in a high thermal resistance and poor performance at elevated temperature. What is needed is an alternative structure for GaN devices such as transistors and diodes that reduces the cost and complexity of die assembly without introducing the aforementioned problems.

SUMMARY

The present disclosure relates to a power device and power device packaging. Generally, the power device of the present disclosure includes a die backside and a die frontside. A semi-insulating substrate with epitaxial layers disposed thereon is sandwiched between the die backside and the die frontside. Bond pads on the die frontside are coupled to the die backside with patterned backmetals that are disposed within vias that pass through the semi-insulating substrate and epitaxial layers from the die backside to the die frontside.

One embodiment includes a power module substrate adhered to the die backside. The power module substrate has an isolation region that electrically isolates patterned backmetals from each other.

Another embodiment has a thermal shunt that is fabricated within the power module substrate between isolation regions. The thermal shunt conducts heat away from the semi-insulating substrate, and in turn away from the epitaxial layers.

Yet another embodiment includes additional circuit element(s) coupled between the patterned backside metals. The additional circuit element(s) can be passive circuit elements or active circuit elements. The additional circuit element(s) can also be limiters and/or protectors for limiting current and overvoltage surges.

Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

FIG. 1 is a cross-section view depicting a bond pad configuration for a prior art power transistor.

FIG. 2 is a plan view of a prior art bond pad layout for gallium nitride GaN high electron mobility transistors (HEMTs).

FIG. 3 is a cross-section view of an exemplary GaN device depicting source and drain connections on a backside of a die in accordance with the present disclosure.

FIG. 4 is a cross-section view of the exemplary GaN device depicting a thermal shunt in addition to the source and drain connections.

FIG. 5 is a cross-section view of the exemplary GaN device depicting an additional circuit element coupled between the drain, source, and/or gate.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

It will be understood that when an element such as a layer, region, or substrate is referred to as being “over,” “on,” “in,” or extending “onto” another element, it can be directly over, directly on, directly in, or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over,” “directly on,” “directly in,” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. Moreover, the term high resistivity and the term semi-insulating are used interchangeably throughout the disclosure. It is also to be understood that semi-insulating means electrically semi-insulating.

FIGS. 3 through 5 depict alternative structures for GaN devices such as transistors and diodes that reduce the cost and complexity of die assembly without introducing the aforementioned problems. Beginning with FIG. 3, a GaN device 42 includes a source pad 44 and a drain pad 46 disposed onto a die frontside 47 that includes GaN epitaxial layers 48. A device active area 50 is located between the source pad 44 and the drain pad 46. A semi-insulating substrate 52 supports the GaN epitaxial layers 48. The semi-insulating substrate 52 has a bulk resistivity that ranges from around about 10⁷ Ohm-cm to around about 10¹² Ohm-cm.

A die backside 54 includes a first patterned backmetal 56 that is disposed within a source backside via 58. The first patterned backmetal 56 couples the source pad 44 to a first die attach region 60 that adheres a power module substrate 62 to the die backside 54. The die backside 54 also includes a second patterned backmetal 64 that is disposed within a drain backside via 66. The second patterned backmetal 64 couples the drain pad 46 to a second die attach region 68 that adheres the power module substrate 62 to the die backside 54. An isolation region 70 provides electrical isolation between the first patterned backmetal 56 and the second patterned backmetal 64.

FIG. 4 is a cross-section view of another embodiment of the present disclosure, wherein a GaN device 72 includes a thermal shunt 74 located within the power module substrate 62 and under the device active area 50 for shunting heat away from the semi-insulating substrate 52. A third patterned backmetal 76 is disposed onto a third die attach region 78, which in turn is disposed onto the thermal shunt 74. A first isolation region 80 and a second isolation region 82 electrically isolate the third patterned backmetal 76, the third die attach region 78, and the thermal shunt 74 from the first patterned backmetal 56 and the second patterned backmetal 64.

FIG. 5 is a cross-section view of another embodiment of the present disclosure, wherein a GaN device 84 includes at least one circuit element 86 that is coupled between the source pad 44 and the drain pad 46 by a direct coupling of the circuit element 86 to the first patterned backmetal 56 and the second patterned backmetal 64. Exemplary types of circuit elements for circuit element 86 include transistors, diodes, resistors, capacitors, and inductors. The circuit element 86 can also be a limiter or protector for preventing damage to the GaN device 84 due to overcurrent, overvoltage, and surge voltage and/or surge current. The circuit element 86 can also be coupled between either the first patterned backmetal 56 or the second patterned backmetal 64 to a feature such as a gate pad (not shown). Moreover, it is to be understood that the circuit element 86 can be also be coupled to other features of the GaN device 84 using other patterned backmetal features like the first patterned backmetal 56 and the second patterned backmetal 64.

Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. A power device with packaging comprising: a die frontside; plurality of bond pads disposed on the die frontside; a die backside; a semi-insulating substrate with at least one epitaxial layer disposed thereon being sandwiched between the die frontside and the die backside; and patterned backmetals disposed within vias that pass through the semi-insulating substrate and the at least one epitaxial layer from the die backside to the die frontside, wherein the patterned backmetals couple the plurality of bond pads to the die backside.
 2. The power device with packaging of claim 1 further including a power module substrate adhered to the die backside.
 3. The power device with packaging of claim 2 further including an isolation region that provides electrical isolation between the patterned backmetals.
 4. The power device with packaging of claim 2 further including a thermal shunt fabricated within the power module substrate for shunting heat away from the semi-insulating substrate.
 5. The power device with packaging of claim 2 further including a circuit element that couples between select ones of the plurality of bond pads.
 6. The power device with packaging of claim 5 wherein the circuit element is a passive device.
 7. The power device with packaging of claim 5 wherein the circuit element is an active device.
 8. The power device with packaging of claim 5 wherein the circuit element is a current limiter.
 9. The power device with packaging of claim 5 wherein the circuit element is an overvoltage protector.
 10. The power device with packaging of claim 1 wherein the epitaxial layers are gallium nitride (GaN) layers.
 11. A method of fabricating a power device with packaging comprising: providing a semi-insulating substrate with at least one epitaxial layer disposed thereon and sandwiched between a die frontside and a die backside; fabricating a plurality of bond pads on the die frontside; fabricating vias through the semi-insulating substrate and the at least one epitaxial layer to create an open path between the die backside to the die frontside beneath the plurality of bond pads; and disposing patterned backmetals within the vias to couple the plurality of bond pads to the die backside.
 12. The method of claim 11 further including adhering a power module substrate to the die backside.
 13. The method of claim 12 wherein the power module substrate includes an isolation region that provides electrical isolation between the patterned backmetals.
 14. The method of claim 12 wherein a thermal shunt is fabricated within the power module substrate for shunting heat away from the semi-insulating substrate.
 15. The method of claim 12 further including coupling a circuit element between select ones of the plurality of bond pads.
 16. The method of claim 15 wherein the circuit element is a passive device.
 17. The method of claim 15 wherein the circuit element is an active device.
 18. The method of claim 15 wherein the circuit element is a current limiter.
 19. The method of claim 15 wherein the circuit element is an overvoltage protector.
 20. The method of claim 11 wherein the epitaxial layers are gallium nitride (GaN) layers. 